Fibre lasers are finding wide commercial application in many industrial processes such as marking, cutting, welding and brazing. Diode-pumped fibre lasers have become commercially available that can emit 10 W to 2 kW of laser radiation in a near diffraction limited beam. These lasers are based on rare-earth doped optical fibres which emit in the fundamental mode. The optical fibres are typically based on large mode area fibres such as those described in U.S. Pat. No. 6,614,975. The large mode area enables output power levels to be increased to levels which would cause optical damage to optics and optical fibres within the laser if conventional singlemode fibres were used, and which would result in non-linear effects such as stimulated Raman scattering and stimulated Brillouin scattering if a truly single-mode core were used.
Further increases in output power can be achieved by relaxing the requirement that the fibre laser emits a near diffraction limited beam. A few-moded fibre laser (beam quality M2 in the range 2 to approximately 20) can be designed by increasing the core diameter. This enables the output power to be increased, whilst maintaining beam qualities that are better than competing technologies. Further increase in power can be achieved by combining the single-mode or few-moded laser radiation from several rare-earth doped fibres together. Rare-earth doped fibre lasers are commercially available that can emit up to 1 kW to 50 kW of laser radiation with excellent beam quality.
Fibre lasers typically use cladding-pumping in which pump light is coupled into the inner cladding of an optical fibre that comprises a rare-earth doped core, an inner cladding, and an outer cladding. The pump radiation is guided along the inner cladding and is gradually absorbed by the core. Cladding pumping has a major advantage in that pump light can be supplied by low-cost, high-power, multi-mode laser diodes. A disadvantage is that the length of fibre required to absorb the pump radiation is longer than a core-pumped fibre laser by a factor approximately equal to the ratio of the area of the inner cladding to the area of the core. This increase in length is undesirable because it reduces the efficiency of a fibre laser or amplifier, and increases undesirable optical non-linear effects.
It is generally desirable for an amplified optical signal to have high beam quality and high peak powers for optimum materials processing capability. In an optical fibre, high beam quality places certain limitations on the dimensions of the core in order to maintain single-moded or few-moded propagation. These restrictions in core size lead to high optical intensities within the core during the amplification of high-power optical radiation. High optical intensities lead to undesirable optical non-linearities which need to be mitigated by short fibre lengths to minimise the interaction length. For example, pulsed fibre lasers having average powers of 10 W to 50 W typically have peak powers in excess of 5 kW. Reducing the fibre length is important to avoid stimulated Brillouin scattering and stimulated Raman scattering. The first can lead to undesirable pulsing and catastrophic failures owing to the associated acoustic phonon shattering the core of the fibre. The latter can lead to undesirable wavelength shifts. Similar non-linear effects are also seen in high-power continuous-wave lasers where power levels in excess of 100 W can lead to wavelength shifts induced by stimulated Raman scattering. The problem is clearly even more important between approximately 400 W to 2 kW in single mode lasers, and between approximately 4 kW to 50 kW in few-moded or multi-moded lasers.
In order to minimise the length of the optical fibre, and thereby minimize losses and non-linear effects, it is preferable to use a pump radiation wavelength that is well-matched to the strongest absorption peak of the active dopant in the core. A commonly-used rare-earth dopant is ytterbium, which has an absorption peak at 976±3 nm. This absorption peak has approximately two to three times the absorption per unit length than the absorption between 910 and 970 nm. However, multi-mode pump laser diodes have poor wavelength repeatability from device-to-device (typically ±10 nm), a strongly temperature-dependent wavelength (typically 0.3 nm/K), and a strong dependence of wavelength on output power (typically 1 nm/W). Over a typical operating temperature range for an industrial laser (0 to 60 C), the wavelength emitted by the pump diodes may vary by as much as 30 nm. Even if the laser diode is temperature stabilised, the power-dependence of wavelength (10 nm for a 10 W emitter) makes it difficult to directly-pump the strongest absorption peak of ytterbium, especially in pulsed applications in which the pump diodes are switched on and off repeatedly. Switching the pump diodes on and off repeatedly leads to pump wavelength variation while the pump diodes thermally stabilise each time they are turned on, and pump wavelength variation as the average temperature of the pump diodes thermally stabilises. Similar limitations occur with other rare earth dopants, and for fibres containing two or more rare-earth dopants, for example, erbium ytterbium fibres which are optimally pumped at 976 nm and which emit at around 1550 nm to 1560 nm. The difficulty in directly pumping the strongest absorption peak of rare earth dopants, cheaply and reliably, provides a limitation to the peak power capability of low-cost rare-earth-doped fibre lasers and amplifiers, and in particular pulsed rare-earth doped fibre lasers and amplifiers. It is for these reasons that the vast majority of high power ytterbium doped fibre lasers and amplifiers use multimode laser diodes emitting pump radiation within a wavelength range of 910 nm to 950 nm at which the absorption of ytterbium is approximately two to three times smaller than at 976 nm.
Recent approaches to stabilizing the pump wavelength include using volume Bragg gratings to provide feedback to the multi-mode laser diodes in order to provide some degree of wavelength-locking. Volume Bragg gratings are expensive, and are typically used on high-current laser diode bars as opposed to the single emitter laser diodes commonly used in fibre lasers. The wavelength locking performance over typical temperature and operating power ranges is also questionable.
There is therefore a need for a laser apparatus in which the laser radiation can be matched to the peak absorption of an active medium. An associated need which is important for environmental (ie green) considerations is to improve the efficiency of lasers and amplifiers. A further need is to allow the increase of optical power from fibre lasers and amplifiers without incurring undesirable non-linear effects.
An aim of the present invention is to provide laser apparatus which reduces the above aforementioned problems.
The Invention:
According to a non-limiting embodiment of the invention, there is provided laser apparatus comprising a reference source, a reference fibre, and at least one laser diode, wherein the reference fibre comprises a core having a refractive index n1 and a first cladding having a refractive index n2, the first cladding is surrounded by a second cladding having a refractive index n3, the refractive index n1 is greater than the refractive index n2, the refractive index n2 is greater than the refractive index n3, the laser diode emits laser radiation that is guided through the first cladding of the reference fibre, the reference source emits reference radiation that has a predetermined wavelength, the reference radiation is guided through the core of the reference fibre to the laser diode, and the reference radiation that is guided through the core of the reference fibre to the laser diode has a power at the predetermined wavelength, which power is greater than an injection locking threshold of the laser diode thereby to injection lock the laser diode.
Injection locking is a process whereby the output frequency of a first oscillator is controlled by coupling radiation from a second, usually more stable oscillator into the first oscillator. In general, laser diodes have poorly defined wavelengths, that are temperature dependent, and which vary as the laser diode is switched on. By coupling the reference radiation from the reference source into the laser diode, it is possible to injection lock the laser diode such that its output wavelength is forced to become substantially equal to the wavelength of the reference radiation. This enables the wavelength of the laser diode to be determined by selecting a reference source that emits at the predetermined wavelength. It also enables the wavelength of the laser diode to be stabilized as it establishes thermal equilibrium shortly after it is turned on.
The predetermined wavelength is a wavelength that is preselected by a user. The predetermined wavelength may be the wavelength at which an amplifier or a laser can be pumped by the laser diode at maximum efficiency, lowest noise, maximum power, shortest length, lowest non-linear phase, or another parameter that has importance to the user.
Advantageously, the invention provides a means to injection lock the laser diode, and in particular a multimode laser diode, rapidly, and simply, and to do this while providing high levels of output powers (greater than 60% of the power emitted by the laser diode, preferably greater than approximately 90%, and more preferably greater than 95%) at a laser output with a wide selection of injection locked wavelengths that are defined by the choice of the predetermined wavelength of the reference source. Suitable predetermined wavelengths include: the peak absorption wavelength of a rare-earth doped fibre laser, rod laser, or disk laser; a wavelength at which heat dissipation is reduced or preferably minimized in a fibre laser, rod laser, or disk laser; and a wavelength which optimizes efficiency within a fibre laser, rod laser, or disk laser. These features are consistent with achieving a fast modulation rate of wavelength-locked pump radiation in fibre lasers, rod lasers and disk lasers, which is necessary for rapid process control in marking, cutting, welding and brazing applications. Moreover, it allows such fast modulation rates to be achieved with increased efficiencies, reduced amplifier fibre lengths, which combination leads to lower non-linear effects (such as self phase modulation) and/or higher peak powers being available from amplifiers and lasers. The invention solves the problem of poor wavelength control and repeatability of multimode laser pump diodes, namely poor wavelength repeatability from device to device±10 nm, strong temperature dependent wavelength (0.3 nm/K), and strong power-dependence on wavelength (1 nm/W). The invention permits amplified optical signals to have high beam quality and high peak powers for optimum materials processing capability without necessitating the high optical intensities within the core during the amplification of high-power optical radiation. It permits shorter fibre lengths to be used to avoid undesirable optical non-linearities. It is particularly useful for reducing pulse distortion owing to self phase modulation and stimulated Brillouin scattering in pulsed fibre lasers having average powers of 10 W to 50 W and peak powers in excess of 5 kW. It is also useful for reducing non-linear effects such as stimulated Raman scattering in high-power single moded or few moded (beam quality M2 in the range 2 to approximately 20) continuous-wave lasers having power levels in excess of 100 W, 400 W, 1 kW or more preferably, in excess of 4 kW.
The reference source can be a semiconductor laser having an external cavity or which is stabilized by an etalon or grating. The reference source can also be a different type of laser such as a gas laser or a solid state laser.
The reference source may be such that the product of its power at the predetermined wavelength and a first loss experienced by the reference radiation in propagating from the reference source to the laser diode is at least 0.5% of the power of the laser radiation emitted by the laser diode. This is to ensure that there is sufficient of the reference radiation incident on the laser diode to injection lock it reliably and repeatedly. Preferably the laser apparatus is designed to minimize the first loss in order to maximise the reference radiation received by the laser diode.
The laser apparatus may be one in which a second loss experienced by the laser radiation in propagating from the laser diode to the reference source is such that the product of the second loss and the power emitted by the laser diode is less than 10% of the power emitted by the reference source. This is to ensure that the laser radiation does not damage the reference source. Preferably the laser apparatus is designed to increase or maximise the second loss in order to minimize the laser radiation received by the reference source.
The laser apparatus may include a coupler for coupling the reference radiation from the core to the first cladding of the reference fibre. The coupler may be a blazed fibre Bragg grating.
The laser apparatus may include a wavelength division multiplexer.
The laser apparatus may include an amplifying means. The amplifying means can be an amplifying fibre.
The reference fibre and the amplifying fibre may be in optical contact along a portion of their length such that the laser radiation guided by the first cladding of the reference fibre can couple into and pump the amplifying fibre.
The amplifying fibre may comprise a pedestal.
The amplifying fibre may comprise at least one rare-earth dopant.
The laser diode may be arranged to counterpump the amplifying fibre.
The reference fibre may be a single mode fibre.
The laser diode may be a multimode laser diode. The multimode laser diode may be a single emitter laser diode, a diode bar, or a diode stack.
The laser apparatus may comprise a plurality of the laser diodes, wherein the laser diodes are combined by a combiner.